Filaments of semimetals such as carbon, boron, silicon carbide, and natural and synthetic polymers, such as cotton, rayon, wool, nylon, polyesters, aramids, and the like, in the form of yarns, mats, cloths and chopped strands, woven, knitted, unidirectional, and other engineered fabric structures are known to be useful in reinforcing metals and plastics. Articles comprising matrix metals reinforced with such filaments are finding widespread use in replacing heavier components made from lower strength conventional materials such as aluminum, steel, titanium, lead, tin, zinc, alloys thereof, and the like in aircraft, aerospace, automobiles, office equipment, sporting goods, etc., and in many other fields.
A common problem in the use of such filaments is a seeming lack of ability to translate the properties of the high strength filaments to the matrix metal to which ultimate and intimate contact is to be made.
The problem is manifested in a variety of ways: for example, if lengths of high strength carbon fiber yarn are enclosed lengthwise in the center of rods formed from solidified molten lead, zinc, or tin, and the rods are pulled until broken, the breaking strengths will be less than those expected from the rule of mixtures, and greater than those of rods formed, respectively, from lead, zinc or tin alone, due to merely mechanical entrapment of the fibers. The lack of reinforcement is entirely due to poor translation of strength between the fibers and the matrix metal. Other fibers, such as rayon fibers in low melting metal matrixes also suffer from such shortcomings.
In R. V. Sara, U.S. Pat. No. 3,622,283, it has been reported that this problem can be overcome by first coating the filaments, e.g., carbon filaments, with a thin, e.g., 1 to 3 microns thick, and continuous metallic film or coupling agent, e.g., nickel, which has a melting point greater than that of the matrix metal, e.g., a tin alloy, and is also readily wetted by the matrix metal. More specifically, short lengths, 4 inch or so, of carbon filaments were clamped in a battery clip, immersed in an electrolyte and nickel plated. Then plated filaments were cut into 1 inch lengths and manually loaded into a perforated tube and vacuum infiltrated with molten tin, then permitted to cool. Not only is such a composite difficult to characterize physically, but also it is of limited use in the production of a wide range of useful workpieces.
Methods for producing metal composites are also described in Evans et al, U.S. Pat. No. 3,550,247, which subjects carbon filaments to an oxidizing treatment under strong oxidizing conditions, e.g., with concentrated nitric acid or chromic acid before coating them with metal e.g., nickel and dispersing them in a matrix of the same, or different metal, e.g., nickel, chromium, aluminum, copper, or lead. The strong oxidizing conditions, however, may be responsible for the observation by others of higher strengths in unplated filament reinforced composites than in plated filament reinforced composites.
In applicant's earlier filed copending application, Ser. No. 358,637, filed Mar. 16, 1982, it is disclosed that, if a very high order of external voltage is applied during electroplating, by special techniques, uniform, continuous, adherent thin, metal coatings can be provided to reinforcing filaments, especially carbon filaments. Coating thickness of 0.25 to 0.75 microns can be achieved. If a core material having a boundary layer, is used, such as carbon filaments, high voltage is used to provide energy to uniformly nucleate the metal ion on the surface, and through the boundary layer. On the other hand, if an electrically conductive core, free of boundary layer, is used, such as roughened aramid fiber chemically plated with adherent silver, then the high voltage is needed to uniformly nucleate the filaments, especially the innermost filaments in bundles or tows of the filaments. Filaments comprising the thin metal coatings or yarns, woven cloths, and the like, according to the procedure disclosed, can be knotted and folded without the metal substantially flaking off.
Articles made by adding metal coated filaments of the above-mentioned copending invention to a metal matrix forming material distinguish those from the prior art, including the above-mentioned U.S. Pat. No. 3,622,283, and U.S. Pat. No. 3,550,247 because they are very strongly reinforced, showing tensile strengths much closer to those which would be predicted from the rule of mixtures.